1. Field of the Invention
The present invention relates to an ink jet recording head used for a recording apparatus in which a liquid ink or the like is discharged on a recording medium and to an ink jet recording apparatus.
2. Description of the Related Art
Generally, in accordance with the increase in recording speed, the number of discharge ports of an ink jet recording head for each ink color has increased to 128 or 256. These discharge ports are arranged so as to realize a high density recording of 300 dpi (dots per inch, a number of dots produced in 1 inch) or 600 dpi, etc. A heating device (i.e., an electrothermal conversion device) provided at the discharge port responds to a pulse drive of several to several tens of microseconds to form a bubble by film boiling. Since the heating device is driven by high-frequency, high-speed printing with high-image quality can be achieved.
In order to achieve high-quality color recording equivalent to that of silver halide photo by using an ink jet recording head, it is necessary to make dots extremely small to an unnoticeable level (i.e., no grainy effect is observed). A droplet of color ink used for ink jet recording has become so small, to approximately 5 pl (picoliter, 10−12 liter) to 2 pl which enables print resolution of 600×1200 dpi to 1200×1200 dpi.
In order to implement a recording head which can form a high gradient image with fine-image quality employing dots of different sizes, US Patent Application No. 2002/196309 discloses a recording head which has more nozzles for discharging small dots than those for discharging large dots. Japanese Patent Application Laid-open No. 2000-141714 also discloses a technique for down-sizing a head without significantly sacrificing image quality or print speed by reducing only the number of nozzles for yellow color and by lowering print resolution for yellow.
As described above, when an image is formed only by large ink drops, it is difficult to obtain high-quality image prints due to grainy images. By making the ink drops smaller, high-quality image printing with no grainy images can be obtained.
However, when the discharge port is made smaller to obtain the fine ink droplet, an amount of ink mist increases compared to a case where a larger dot is used. The ink mist is a plurality of fine ink droplets, called a satellite which is discharged from the discharge port of the recording head together with the main ink drop, or extremely small droplets of spattered ink which are produced when the main drop hits a recording medium.
Smaller ink droplets increase the amount of ink mist, thereby the ink mist can adhere to the discharge port of the recording head. The adhering ink mist produces an ink pool at the discharge port of the head, which may result in discharge failure (see FIG. 17). Especially when high-duty discharge is performed, viscous air at the discharge port group is pulled along with the discharge of the ink drop from the discharge port onto the recording medium.
As a result, pressure in the vicinity of the discharge port becomes lower than that at the periphery of the recording medium, and the air at the periphery flows into the low-pressure area (in the vicinity of the discharge port area), so that airflow can rise from the recording medium. Due to this airflow, a large amount of mist adheres to the discharge port and can cause discharge failure. Further, an error can also occur due to an increased amount of ink mist adhering to a sensor or a scale which is used in detecting the position of a carriage mounted with the recording head.
In addition, when the ink droplet becomes so fine, a large amount of ink is required in order to print an image compared to a case where a large dot is used since the image area has to be covered with fine dots.
An example of a conventional recording head includes the same discharge port configuration for each color of cyan, magenta, or yellow ink with large and small discharge ports as shown in FIG. 18.
FIG. 19 is a graph showing ink color in relation to the number of dots printed on a recording medium when printing a portrait photo, a popular item for ink jet printers, by using the conventional recording head. In FIG. 19, 2 pl corresponds to a small discharge port and 10 pl to a large discharge port.
A photo-quality print has a large image area that is printed with small dots discharged from small discharge ports to make the dots unnoticeable, i.e. without grains. If small dots are used to cover a certain size of a recording medium, a greater number of them is required as compared to a case where larger dots are used. Therefore, the number of small dots discharged from small discharge ports tends to increase. Additionally, since a yellow ink, which is brighter than other inks, is frequently used in printing lighter areas of an image such as human skin, the number of yellow dots discharged from small discharge ports further increases.
A recording head provided with the small discharge port discharges a greater number of dots as compared to discharges made by only a large port, and an electrothermal conversion element (i.e., a heating device) heated each time the port discharges. Not all the thermal energy generated by the heating device is converted to blowing energy and some of the energy tends to accumulate in the recording head. The head temperature rises accordingly.
Once the head temperature reaches a certain level due to continuous ink discharge, viscosity of the ink lowers and the bubble size becomes larger than the appropriate size. Then, the discharge becomes unstable which could cause phenomena such as a streak or an uneven print. Conventional techniques therefore reduce the print speed according to the rise in head temperature, however, such technique hinders improvement of throughput.
Further, since the amount of discharged ink increases as the ink droplet becomes small, there is concern about mechanical damage to the electrothermal conversion element due to cavitation produced by repeated bubbling and debubbling of ink. Besides, repeated application of pulse energy generates heat which can damage the heating device of the electrothermal conversion element. This causes discharge failure that leads to white streaks.